WooCommerce - Disable VAT calculation in backend

/**
 * Aelia EU VAT Assistant - Disable tax calculations on the backend for orders that have a "VAT number
 * validated" meta saved against them.
 *
 * HOW TO USE THIS CODE
 * Simply add the code to the bottom of your theme's functions.php file, and it
 * will run automatically. For more information: https://www.skyverge.com/blog/add-custom-code-to-wordpress/
 *
 * GPL DISCLAIMER
 * Because this code program is free of charge, there is no warranty for it, to the extent permitted by applicable law.
 * Except when otherwise stated in writing the copyright holders and/or other parties provide the program "as is"
 * without warranty of any kind, either expressed or implied, including, but not limited to, the implied warranties of
 * merchantability and fitness for a particular purpose. The entire risk as to the quality and performance of the program
 * is with you. should the program prove defective, you assume the cost of all necessary servicing, repair or correction.
 *
 * Need a consultation, or assistance to customise this code? Find us on Codeable: https://aelia.co/hire_us
 */

/**
 * Intercepts the "Recalculate" Ajax call performed by WooCommerce in the backend,
 * when the Recalculate button is clicked, to disable tax calculations for orders
 * that have a valid VAT number associated to it.
 *
 * IMPORTANT
 * This code only checks the value of the '_vat_number_validated' meta. The
 * following aspects are not covered:
 * - Changing the billing country.
 * - Changing the VAT number.
 * - Tax recalculations triggered by operations other than the "Recalculate" button on the Edit Order page.
 *
 * Selecting a different country or VAT number would require a new validation.
 * The EU VAT Assistant is designed to validate the VAT number on the frontend,
 * and won't take any actions for orders created or modified in the backend. Due
 * to that, changing country or VAT number won't affect the data already collected,
 * or the value of the VAT exemption meta.
 */
add_action('wp_ajax_woocommerce_calc_line_taxes', function() {
  // If the EU VAT Assistant is not active, do nothing
  if(empty($GLOBALS['wc-aelia-eu-vat-assistant'])) {
    return;
  }

  // Retrieve the order ID
  $order_id = absint($_POST['order_id']);
  if(!is_numeric($order_id)) {
    return;
  }

  // Retrieve the value of the "vat_number_validated" meta
  $order = wc_get_order($order_id);
  $vat_number_validated = $order->get_meta('_vat_number_validated');

  // If the VAT number was validated, disable the tax calculation
  // altogether. This is necessary because WooCommerce doesn't allow
  // to set a VAT exemption on the customer, during this operation
  if($vat_number_validated === 'valid') {
    add_filter('wc_tax_enabled', function($tax_enabled) {
      return false;
    });
  }
}, 1, 1);